The present invention relates to glass producing apparatus, and more particularly to improved means for supplying vaporized source material to an apparatus for forming blanks to be drawn into optical waveguides.
Light can be caused to propagate within an elongate transparent body (such as a filament of glass or the like) in discrete modes if certain preconditions are met. The size of the filament, the radial gradation in refractive index, and other considerations combine to determine the effectiveness of the filament as a transmitting medium for optical communications. In order to transmit the light without excessive dispersion among propagation modes, or to allow only predetermined modes of light to propagate, the internal characteristics of the strand must be closely controlled.
It is well known that a filament suitable for use as an optical waveguide can be formed by heating a cylindrical blank of a transparent dielectric material, such as glass, and drawing the blank into the desired thin, elongate structure. The structural characteristics of the waveguide closely reflect those of the blank from which it is drawn, particularly the gradient of the refractive index.
According to one popular practice a drawing blank is formed by coating a rotating, cylindrical starting member with successive layers of a sinterable glass soot. The soot is built up, layer by layer, from minute siliceous particles which are applied to the surface of the blank through the mechanism of an oxidizing reaction flame or the like. As will be recognized by those skilled in the art, the flame used for sintering and transferring particles to the surface of a glass blank or the like has in the past been termed a "hydrolyzing flame". Although the precise phenomenon involved is not yet fully understood, recent studies suggest that the actual reaction is more properly characterized as oxidation. Accordingly, the description of this reaction with respect to a presently preferred embodiment will use the term "oxidizing", it being recognized that the precise nature of the chemical reaction involved is not a material factor in practicing the present invention. A siliceous matrix material, such as silicon tetrachloride, is supplied in the form of a vapor to a burner from which the flame issues. Other materials, herein termed dopants, are also supplied in controlled amounts and at particular times to vary the optical characteristics of the end product. It should be understood that whereas the preferred embodiment is described in conjunction with the formation of silicate glasses, silica-free glasses containing germania, for example, may be formed by the apparatus of this invention.
The various vaporized or nebulized materials then combine with oxygen in the burner flame to form tiny spherical particles, which are maintained in the molten state and propelled toward the surface of the blank by the force of the flame. In this manner the deposited material, commonly termed "soot", is laid down along a spiral locus, layer by layer, the various layers merging together to form a continuum.
In another well known process the vaporous source material is directed into a heated tube. The material is deposited within the tube in successive layers and the tube is then collapsed to leave a fused blank. The blank is subsequently heated and drawn into an elongate filament.
In order to form a blank having generally consistent properties, and to assure an even distribution of the glassforming soot, it has been necessary to supply a hydrolyzing burner with a substantially constant flow of vaporized source material entrained in a carrier gas. Accordingly, systems have been devised for controlling the carrier gas flow, and the rate at which source material is vaporized and entrained into the carrier gas. Due in part to the multiplicity of factors which determine the amount of vapor introduced into a hydrolyzing flame at any given time, fluctuations in the deposited rate of the various vapor deposition processes often occur.
An improved system that employs a carrier gas to deliver a reactant vapor is disclosed in copending application Ser. No. 872,619 filed in the name of M. Aslami on Jan. 26, 1978 and assigned to the assignee of the present application. That system comprises a column having a packing therein and a pump for circulating liquid source material through the packing. Pressurized carrier gas is introduced into the packing and flows therethrough, the source material vapor becoming entrained in the carrier gas. The gas leaves the column through an exhaust port and is conducted to the site at which it is to be deposited. Although the vapor flow rate can be accurately controlled by the system of the Aslami application, the pressure and temperature within the liquid source material reservior must be precisely controlled. Accordingly, it will be appreciated that it would be highly desirable to provide a system for delivering vaporized source material to a hydrolyzing burner in a consistent, highly controllable fashion without having to precisely control the temperature and pressure in the source material reservoir.
My copending application Ser. No. 885,215 filed Mar. 10, 1978 and assigned to the assignee of the present application discloses an apparatus which supplies a precisely controlled amount of reactant vapors to discharge means. In accordance with the teachings of that application, various reaction product constituents are maintained in liquid form in reservoirs, and a metering pump is coupled to each reservoir for delivering a predetermined volume of the liquid constituent to a mixing stage. The various liquids are thoroughly mixed, then nebulized. The material vapors are transported to discharge means such as a reaction burner or the like. The source material vapors are then reacted in the flame in a conventional manner, and deposited upon a substrate to form a product such as an optical waveguide blank. Although the apparatus of my copending application is capable of precisely controlling the amount of the various reaction product constituents delivered to the utilization means, the variable output pumps utilized in that system are very expensive.
From the foregoing, it should now be understood that it would be highly desirable to provide a system for delivering waveguide materials to a reaction burner or the like which obviates the above disadvantages, and provides a controllable flow of highly densified vapors of the desired materials.
It is therefore an object of the present invention to provide improved apparatus for supplying vaporized source material to a utilization means.
A further object of the invention is to provide a system for delivering waveguide constituents from their respective reservoirs without the need for a carrier gas.
Another object is to furnish simplified apparatus for metering waveguide component materials.
Yet another object is to provide an apparatus capable of delivering increased amount of reactant vapors to a vapor deposition means.